With rapid development of network techniques and popularization of network applications, for example, network communications, online shopping, network entertainment and the like have become a part of modern people's life. An existing access network copper wire (wired) system can not meet the requirement of this high speed and broad band. However, a PON which has advantages of broad band, high speed, environment friendliness and energy conservation is a best option for replacing the existing access network. At present, the PON is being accepted and provided by most operators for satisfying growing communication users and meeting faster and better service requirements.
The PON is a point-to-multipoint fiber access technology. As shown in FIG. 1, the PON comprises an Optical Line Terminal (OLT), an Optical Network Unit (ONU) and an Optical Distribution Network (ODN), wherein the point-to-multipoint structure of the PON generally is formed by coupling one OLT with a plurality of ONUs through an optical power splitter (optical splitter for short) of the ODN.
After the setup and deployment of a large number of PONs, it is needed to consider the operation and maintenance of the network, especially detection of a fiber line and location of a fault. In order to reduce operation and maintenance costs, operators expect to adopt at the OLT an optical path detection device, also called an Optical Time Domain Reflectometer (OTDR), to detect a trunk fiber and branch fibers of the entire PON. If one branch fiber has a fault, it is expected to find and locate the fault and to perform maintenance quickly without influencing services of other branch fibers.
When one OTDR is adopted at a central office OLT to detect this point-to-multipoint network, it can be accurately detected whether the trunk fiber is normal; however, when detecting a signal of the branch fiber, two problems as follows would be encountered.
1. If part branch fibers have approximately equal distance to the optical splitter, the OTDR can not distinguish which branch fiber the signal is from, except that a high resolution OTDR is adopted. However, the highest resolution that can be provided at present is 2 meters, which still can not meet the actual needs.
2. If a splitting ratio of the optical splitter is very large, a Rayleigh reflection signal of the branch fiber would suffer great loss when passing through the optical splitter. When the reflection signal arrives at the OTDR, the reflection signal is mixed with noises and is difficult to be distinguished.
For example, for a 10-kilometer ODN with a splitting ratio of 1:32, the loss of the optical splitter is 3*5+3=18 dB, while the loss of a 10-kilometer fiber is 0.40*10=4.0 dB. Generally, the widest dynamic range of the OTDR is about 40 dB. For example, when a signal of the OTDR passes through the optical splitter and reaches a tail end of the branch fiber, and then is totally reflected (that is, reflection loss is not counted) to the OTDR via the optical splitter, if other loss (for example, connection loss) is not counted, then the maximum full path loss of the signal of the OTDR would be 2*18+2*4.0=44 dB, which already exceeds the work dynamic range of the OTDR; therefore, the signal of the branch fiber is submerged in noises. Thus, it can be seen that the conventional OTDR adopted at the central office can not measure the fault of the branch fiber of the ODN with a large splitting ratio. This phenomenon is very popular. In an actual PON network, due to various reasons, even in the PON with a very small splitting ratio, the reflection signal of the branch fiber can not be obtained by a common OTDR.
In view of the problems above, an existing solution is to add one optical filter before each ONU, refer to FIG. 2. This filter transmits all lights with wavelength less than 1625 nm, but reflects lights transmitted by the OTDR with wavelength greater than 1625 nm. After the optical filter is adopted, the light reflected by a port can be enhanced by 6 dB, and in the cooperation of a high-resolution OTDR, it can be determined whether there is a fault in the branch fiber based on whether there are reflected lights. However, when there is a fault in the branch fiber, since the reflected light on the branch fiber is not enhanced by the optical filter, the above phenomenon that the reflected light is submerged in noises still exists; therefore, the exact position of the fault on the branch fiber can not be determined. In addition, if there are part branch fibers having basically equal lengths, the reflected lights basically are overlapped, even the high-resolution OTDR can not distinguish which branch fiber the reflected light is received from. What is worse, for the ODN with a large splitting ratio (for example, greater than 1:128), the gain brought by the filter probably is far less than enough to compensate the loss caused by the optical splitter. Therefore, the OTDR at the central office probably can not receive any information from the branch fiber; consequently, it can not be determined whether there is a fault in the branch fiber and the specific location of the fault can not be determined.